karbokation

8/17/2019 karbokation

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CARBOCATION

REARRANGEMENTS & FREE-

RADICAL ADDITION

Oleh1. Siti Nursiami (4301410002)

2. Ana Yustika (4301410005)

3. Fransisca Ditawati N. (43014100!!)

4. "ut#ia $i%&' Amalia (43014100!!)

Carbocation

Rearrangements

Free-RadicalAddition


2/15

CARBOCATION REARRANGEMENTS

IN HYROGEN HA!IE AITION TO A!"ENES

O#r belie$ t%at carbocations are intermediates in t%e addition o$

%&drogen %alides to al'enes is strengt%ened b& t%e obser(ation

t%at rearrangements sometimes occ#r) For e*am+le, t%e reaction

o$ %&drogen c%loride it% .-met%&l-/-b#tene is e*+ected to

+rod#ce 0-c%loro-.-met%&lb#tane) Instead, a mi*t#re o$ 0-c%loro-

.-met%&lb#tane and 0- c%loro-0-met%&lb#tane res#lts)


3/15

CH2=CHCH(CH3)2 CH3CHCH(CH3)2 + CH3CH2C(CH3)2

Cl Cl

HCl

0oC

.-Met%&l-/-b#tene

0-C%loro-.-met%&lb#tane

12345

0-C%loro-0-met%&lb#tane

16345

Addition begins in t%e #s#al a&, b& +rotonation o$ t%e

do#ble bond to gi(e, in t%is case, a secondar& carbocation)

T%is carbocation can be ca+t#red b& c%loride to gi(e 0-

c%loro-.-met%&lb#tane 12345 or it can rearrange b& a& o$

a %&dride s%i$t to gi(e a tertiar& carbocation) T%e tertiar&

carbocation reacts it% c%loride ion to gi(e 0-c%loro-0-

met%&lb#tane 16345)


4/15

+ + CH3CH —C(CH3)2 CH3CH—C(CH3)2 │

│

H H

Hydride shift

/,0-imet%&l+ro+&l cation

1secondar&5/,/-imet%&l+ro+&l cation

1tertiar&5

T%e similar &ields o$ t%e to al'&l c%loride +rod#cts

indicate t%at t%e rate o$ attac' b& c%loride on t%e

secondar& carbocation and t%e rate o$ rearrangement

m#st be (er&similar)


5/15

FREE-RAICA! AITION

OF HYROGEN BROMIE TO A!"ENES

For a long time t%e regioselecti(it& o$ addition o$ %&drogen

bromide to al'enes as #n+redictable) Sometimes addition

occ#rred according to Mar'o(ni'o(7s r#le, b#t at ot%er

times, seemingl& #nder t%e same conditions, t%e o++osite

regioselecti(it& (anti- Markovnikov addition) was observed. In

1929, Morris S. Kharasch and his students at t%e 8ni(ersit&

o$ C%icago began a s&stematic in(estigation o$ t%is +#99le)


6/15

A$ter %#ndreds o$ e*+eriments, "%arasc% concl#ded t%at

anti-Mar'o(ni'o( addition occ#rred %en +ero*ides, t%at is,

organic com+o#nds o$ t%e t&+e ROOR, ere +resent in t%e

reaction mi*t#re) He and %is colleag#es $o#nd, $or e*am+le,

t%at care$#ll& +#ri$ied /-b#tene reacted it% %&drogen

bromide to gi(e onl& 0-bromob#tane:t%e +rod#ct e*+ected

on t%e basis o$ Mar'o(ni'o(7s r#le)

CH2=CHCH2CH3 + HBr CH3CHCH2CH3

│ Br

no

+ero*ides

/-B#tene H&drogen

bromide

0-Bromob#tane

1onl& +rod#ct;


7/15

On t%e ot%er %and, %en t%e same reaction as +er$ormed

in t%e +resence o$ an added +ero*ide, onl& /-bromob#tane

as $ormed)

CH2=CHCH2CH3 + HBr BrCH2CH2CH2CH3+ero*ides

/-B#tene H&drogenbromide

/-Bromob#tane1onl& +rod#ct;


8/15

"%arasc% termed t%is +%enomenon t%e +ero*ide e$$ect and

demonstrated t%at it co#ld occ#r e(en i$ +ero*ides ere not

deliberatel& added to t%e reaction mi*t#re) 8nless al'enes

are +rotected $rom atmos+%eric o*&gen, t%e& become

contaminated it% small amo#nts o$ al'&l %&dro+ero*ides,

com+o#nds o$ t%e t&+e ROOH) T%ese al'&l %&dro+ero*ides

act in t%e same a& as deliberatel& added +ero*ides to

+romote addition in t%e direction o++osite to t%at +redicted

b& Mar'o(ni'o(7s r#le)


9/15

"%arasc% +ro+osed t%at %&drogen bromide can add to

al'enes b& to di$$erent mec%anisms, bot% o$ %ic% are, in

modern terminolog&, regios+eci$ic) T%e $irst mec%anism is

t%e one e disc#ssed in t%e +receding section, electro+%ilic

addition, and $ol los Mar'o(ni'o(7s r#le) It is t%e

mec%anism $olloed %en care is ta'en to ens#re t%at no

+ero*ides are +resent) T%e second mec%anism is t%e $ree-

radical c%ain +rocess)


10/15

>ero*ides are initiators; they are not incororated into the

roduct but act as a so#rce o$ radicals necessar& to get t%e c%ain

reaction started) T%e o*&gen?o*&gen bond o$ a +ero*ide is

relati(el& ea', and t%e $ree-radical addition o$ %&drogen

bromide to al'enes begins %en a +ero*ide molec#le #ndergoes

%omol&tic clea(age to to al'o*& radicals)

T%is is de+icted in ste+ /, bromine atom is generated in ste+ 0

%en one o$ t%ese al'o*& radicals abstracts a +roton $rom

%&drogen bromide) Once a bromine atom becomes a(ailable,

t%e +ro+agation +%ase o$ t%e c%ain reaction begins) In t%e

+ro+agation +%ase as s%on in ste+ ., a bromine atom adds to

t%e al'ene in t%e direction t%at +rod#ces t%e more stable al'&l

radical)


11/15

T%e o(erall reaction@

CH3CH2CH=CH2 + HBr CH3CH2CH2CH2Br ROOR

light or heat/-B#tene H&drogenbromide

/-Bromob#tane


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THE MECHANISM

a. Initiation

step 1: Dissoiation of a pero!ide into t"o al#o!y

radials:

RO : OR RO + OR . .

. .

. .

. .


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Step 2: Hydrogen atom abstraction from

hydrogen bromide by an alo!y radical:

"#$ H : %r "# H & %r


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(b) Chain propagation

Step ': Addition of a bromine atom to the

alene:

CH3CH2CH=CH2 Br CH3CH2CH2 — CH2 :Br:


15/15

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